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Tungsten and tungsten alloys show the typical change in fracture behavior frombrittle at low temperatures to ductile at high temperatures. In order to improve theunderstanding of the effect of microstructure the fracture toughness of pure tungsten,potassium doped tungsten, tungsten with 1wt% La2O3 and tungsten rheniumalloys were investigated by means of 3-point bending -, double cantilever beam -and compact tension specimens. All these materials show the expected increase infracture toughness with increasing temperature. The experiments demonstrate thatthe grain size, texture, chemical composition, grain boundary segregation and dislocationdensity seem to have a large effect on fracture toughness below the DBTT.These influences can be seen in the fracture behavior and morphology, where twokinds of fracture occur: on one hand the transgranular and on the other hand the intergranularfracture. Therefor techniques like electron backscatter diffraction, augerelectron spectroscopy and x-ray line profile analysis were used to improve the understandingof the parameters influencing fracture toughness.